Multilayer Oxide Ceramic Body With Aligned Sintering Behaviour

Abstract

The invention relates to multilayer oxide ceramic bodies and in particular presintered multilayer oxide ceramic blanks and oxide ceramic green bodies, which comprise at least two different layers and are suitable for dental applications, wherein at least one layer contains La.sub.2O.sub.3 and the at least two different layers differ in their content of La.sub.2O.sub.3. These bodies can be thermally densified by further sintering without distortion and are therefore particularly suitable for the production of dental restorations. The invention also relates to a process for the production of such multilayer oxide ceramic bodies as well as a process for the production of dental restorations using the multilayer oxide ceramic bodies.

Claims

1. Presintered multilayer oxide ceramic blank for the production of dental restorations, which comprises at least two different layers, wherein at least one layer comprises La.sub.2O.sub.3 and the at least two different layers differ in their content of La.sub.2O.sub.3.

2. Blank according to claim 1, in which at least one layer comprises 0.005 to 5 wt.-% La.sub.2O.sub.3.

3. Blank according to claim 1, in which at least one layer comprises Al.sub.2O.sub.3 and/or MgO.

4. Blank according to claim 3, in which at least one layer comprises La.sub.2O.sub.3 and at least one other layer comprises Al.sub.2O.sub.3 and/or MgO.

5. Blank according to claim 3, wherein the at least one layer comprises 0.001 to 5 wt.-% Al.sub.2O.sub.3 and/or MgO.

6. Blank according to claim 3, wherein the at least one layer comprises 0.001 to 5 wt.-% Al.sub.2O.sub.3.

7. Blank according to claim 3, wherein the at least one layer comprises 0.001 to 5 wt.-% MgO.

8. Blank according to claim 3, wherein the at least one layer comprises Al.sub.2O.sub.3 and MgO in a weight ratio of 10:1 to 1:10.

9. Blank according to claim 1, in which at least one layer comprises Y.sub.2O.sub.3 and in an amount of 0.1 to 20.0 wt.-%.

10. Blank according to claim 9, in which at least two different layers differ in their content of Y.sub.2O.sub.3, wherein the difference in the Y.sub.2O.sub.3 content between a layer with the lowest Y.sub.2O.sub.3 content and a layer with the highest Y.sub.2O.sub.3 content is at least 1.0 wt.-%.

11. Blank according to claim 10, in which the layer with the lowest Y.sub.2O.sub.3 content comprises 0.005 to 5 wt.-% La.sub.2O.sub.3.

12. Blank according to claim 10, in which the layer with the highest Y.sub.2O.sub.3 content comprises 0.001 to 5 wt.-% Al.sub.2O.sub.3 and/or MgO.

13. Blank according to claim 9, in which the proportion by weight of La.sub.2O.sub.3 in each of at least two different layers is calculated according to the following formula:
m(La.sub.2O.sub.3)=m.sub.min(La.sub.2O.sub.3)+(m.sub.max(Y.sub.2O.sub.3)m(Y.sub.2O.sub.3))*f, wherein m(La.sub.2O.sub.3) is the proportion by weight of La.sub.2O.sub.3 in the respective layer, m.sub.min(La.sub.2O.sub.3) is the minimum proportion by weight of La.sub.2O.sub.3 of all layers, m(Y.sub.2O.sub.3) is the proportion by weight of Y.sub.2O.sub.3 in the respective layer, m.sub.max(Y.sub.2O.sub.3) is the maximum proportion by weight of Y.sub.2O.sub.3 of all layers, and f is in the range of from 0.01 to 1.00.

14. Blank according to claim 1, in which the oxide ceramic is based on zirconia.

15. Blank according to claim 1, which is suitable for the production of a multi-unit dental restoration.

16. Blank according to claim 1, in which the at least two different layers have different colours.

17. Blank according to claim 1, which has a coefficient of distortion $d=\frac{\left({\mathrm{HV}}_{\max }-{\mathrm{HV}}_{\min }\right)}{\stackrel{\_}{\mathrm{HV}}}$ of less than 0.4, wherein the coefficient is calculated on the basis of at least one measurement of HV for each of the different layers, wherein: HV is the Vickers hardness measured at a load in the range of from 2.5 to 5.0 kgf (24.517 to 49.034 N) in accordance with ISO 14705:2008; HV.sub.max is the maximum of the measured values of HV; HV.sub.min is the minimum of the measured values of HV; and HV is the arithmetic mean of the measured values of HV.

18. Multilayer oxide ceramic green body for the production of dental restorations, which comprises at least two different layers, wherein at least one layer comprises La.sub.2O.sub.3 and the at least two different layers differ in their content of La.sub.2O.sub.3.

19. Green body according to claim 18, wherein at least one layer comprises Al.sub.2O.sub.3 and/or MgO.

20. Blank according to claim 1, in which the sintering behaviour of the at least two different layers is aligned such that the blank can sinter without distortion.

21. Process for the production of a blank according to claim 1, in which (a) at least one first oxide ceramic material and one second oxide ceramic material are provided which differ in terms of their chemical composition, (b) La.sub.2O.sub.3 is added to at least one of the oxide ceramic materials, and (c) optionally, Al.sub.2O.sub.3 and/or MgO is added to at least one oxide ceramic material.

22. Process according to claim 21, in which at least one oxide ceramic material is infiltrated or coated with La.sub.2O.sub.3 and/or at least one oxide ceramic material is infiltrated or coated with Al.sub.2O.sub.3 and/or MgO.

23. Process according to claim 21, in which furthermore (d) layers of the oxide ceramic materials are formed and the layers are arranged one on the other, (e) the oxide ceramic materials are compacted in order to obtain a green body, and (f) optionally, the green body is presintered in order to obtain a presintered ceramic blank.

24. Process according to claim 23, in which the layers of the oxide ceramic materials exhibit a continuous change of composition from a composition of the first oxide ceramic material to a composition of the second oxide ceramic material.

25. Multilayer oxide ceramic blank or green body which is obtainable by a process according to claim 21

26. Process for the production of a dental restoration, in which a blank according to claim 1 is used.

27. Process according to claim 26, in which the blank is given the shape of a desired geometry in order to obtain a shaped ceramic product, wherein the shaping is carried out by machining.

28. Process according to claim 27, in which the shaped ceramic product has the shape of a dental framework or abutment or of a monolithic fully anatomical dental restoration or a multi-unit dental restoration.

29. Process according to claim 27, in which furthermore the shaped ceramic product is densely sintered.

30. Blank according to claim 1, which comprises at least two different layers, which are formed from at least one first oxide ceramic material and one second oxide ceramic material, wherein the layers of the oxide ceramic materials exhibit a continuous change of composition from the composition of the first oxide ceramic material to the composition of the second oxide ceramic material.

Description

EXAMPLES

[0103] General Process for the Treatment of Oxide Powders With Colouring Agents and/or Dopants

[0104] The treatment of oxide powders in the following examples was carried out analogously to US 2014/135200 A1.

[0105] For this purpose, an aqueous treatment solution was prepared, which contained a suitable amount of water-soluble nitrates of the elements with which the oxide powder was intended to be treated. A suitable amount of oxide powder (for example 1,000 g) was introduced into the mixing vessel of an EL1 laboratory mixer (Eirich, Hardheim), which furthermore had an agitator (star-shaped agitator) and a spray nozzle (0.3 mm hollow cone) for applying a solution. The mixing vessel was set in motion at a speed of 13 m/s and the oxide powder located therein was uniformly stirred by the agitator. Then, about 0.1 g treatment solution per 1 g oxide powder were applied to the oxide powder via the spray nozzle with the aid of a 120S/DV-type peristaltic pump (Watson Marlow, Rommerskirchen; speed 170 rpm), thus infiltrating the oxide powder uniformly with the treatment solution.

Example 1

[0106] Aligning the Sintering Behaviour of a Two-Layer Blank Made of Zirconia Powders With Different Yttrium Content by Doping With La.sub.2O.sub.3

[0107] For the preparation of coloured zirconia powders, commercially available zirconia powders (TOSOH TZ-PX-471 and TOSOH Zpex Smile) were used as raw materials and were treated with solutions of nitrate salts of colouring elements and optionally lanthanum as dopant according to the following table using the general process. Thereby, a powder (L1) suitable for a dentine layer and a powder (L2) suitable for a cutting layer were obtained:

TABLE-US-00001 Starting Powder powder Colouring elements .sup.1) Dopant L2 TOSOH 0.028 wt.-% Fe.sup.4) Cutting Zpex Smile.sup.2) 0.0014 wt.-% Cr.sup.5) 0.2283 wt.-% Er.sup.7) L1 TOSOH 0.0091 wt.-% Fe.sup.4) 0.1537 wt.-% La.sup.8) Dentine TZ-PX-471.sup.3) 0.0024 wt.-% Cr.sup.5) 0.002 wt.-% Pr.sup.6) 0.3981 wt.-% Er.sup.7) .sup.1) based on the total weight of the oxide mixture after the sintering .sup.2)contains 9.25 wt.-% Y.sub.2O.sub.3 and 0.048 wt.-% Al.sub.2O.sub.3 .sup.3)contains 7.37 wt.-% Y.sub.2O.sub.3 and 0.048 wt.- % Al.sub.2O.sub.3 .sup.4)calculated as Fe.sub.2O.sub.3 .sup.5)calculated as Cr.sub.2O.sub.3 .sup.6)calculated as Pr.sub.2O.sub.3 .sup.7)calculated as Er.sub.2O.sub.3 .sup.8)calculated as La.sub.2O.sub.3

[0108] Subsequently, about 10 g of each of the coloured zirconia powders was introduced one after the other into the pressing die (diameter about 40 mm) of a laboratory axial press in two layers (at the bottom: L1, on top: L2) and densified axially under a pressure of about 160 MPa. The green body obtained in this way was debound and presintered using the following firing programme:

[0109] 60 K/min to 120 C.,

[0110] 24 K/min to 200 C.,

[0111] 10 K/min to 320 C.,

[0112] 60 K/min to 1050 C., holding time 3 h.

[0113] An about 2 mm thick section was sawn out of the debound and presintered blank obtained in this way using a saw (IsoMet 4000, Buehler, Esslingen). A paper or ruler edge was laid on the two lower outer edges of the section as a reference line and the area of maximum bending was measured under a stereo microscope (SZX 16, Olympus, Hamburg). The results are shown in FIGS. 1A (overview) and 1B (detailed view). The presintered blank showed only a negligible bending of 0.03 mm.

[0114] Finally, the section of the presintered blank was densely sintered in a Programat S1 furnace (Ivoclar Vivadent AG, Schaan) using the following firing and cooling programme:

[0115] 600 K/h to 900 C., holding time 0.5 h

[0116] 200 K/h to 1500 C., holding time 2 h

[0117] 600 K/h to 900 C.,

[0118] 500 K/h to 300 C.

[0119] The section of the densely sintered blank was in turn measured under the stereo microscope as described above. The results are shown in FIGS. 2A (overview) and 2B (detailed view). The densely sintered blank showed no measurable bending.

Example 2

Comparison

[0120] Sintering Behaviour of a Two-Layer Blank Made of Zirconia Powders With Different Yttrium Content Without Doping With La.sub.2O.sub.3

[0121] Example 1 was repeated identically, wherein however no lanthanum was added as dopant. The results for the debound and presintered blank are shown in FIGS. 3A (overview) and 3B (detailed view). Therein, it can be clearly seen that the dentine layer (L1) has shrunk more than the cutting layer (L2), whereby the blank has obtained a convex shape. The bending of the blank was 0.36 mm. The results after the dense sintering of the blank are shown in FIGS. 4A (overview) and 4B (detailed view). Even the densely sintered blank still showed a bending of 0.04 mm thereafter.

Example 3

[0122] Aligning the Sintering Behaviour of a Four-Layer Blank Made of Zirconia Powders With Different Yttrium Content by Doping With La.sub.2O.sub.3

[0123] For the preparation of coloured zirconia powders, commercially available zirconia powders (TOSOH TZ-PX-471 and TOSOH Zpex Smile) were used as raw materials and were treated with solutions of nitrate salts of colouring elements and optionally lanthanum as dopant according to the following table using the general process. Thereby, a powder (L1) suitable for a dentine layer and a powder (L4) suitable for a cutting layer were obtained. By mixing these powders in the ratio 1:2 or 2:1, respectively, using a shaker mixer (Turbula, WAB, Muttenz), two powders suitable for intermediate layers were obtained in addition:

TABLE-US-00002 Starting Colouring Powder powder elements .sup.1) Dopant L4 TOSOH 0.028 wt.-% Fe.sup.4) Cutting Zpex Smile.sup.2) 0.0014 wt.-% Cr.sup.5) 0.2283 wt.-% Er.sup.7) L3 Mixture of 0.0489 wt.-% Fe.sup.4) 0.0512 wt.-% La.sup.8) Intermediate TOSOH Zpex 0.0017 wt.-% Cr.sup.5) layer 2 Smile.sup.2) and 0.0007 wt.-% Pr.sup.6) TOSOH 0.2849 wt.-% Er.sup.7) TZ-PX-471.sup.3) (2:1) L2 Mixture of 0.0698 wt.-% Fe.sup.4) 0.1025 wt.-% La.sup.8) Intermediate TOSH Zpex 0.0021 wt.-% Cr.sup.5) layer 1 Smile.sup.2) and 0.0014 wt.-% Pr.sup.6) TOSOH 0.3415 wt.-% Er.sup.7) TZ-PX-471.sup.3) (1:2) L1 TOSOH 0.0091 wt.-% Fe.sup.4) 0.1537 wt.-% La.sup.8) Dentine TZ-PX-471.sup.3) 0.0024 wt.-% Cr.sup.5) 0.002 wt.-% Pr.sup.6) 0.3981 wt.-%Er.sup.7) .sup.1) based on the total weight of the oxide mixture after the sinterng .sup.2)contains 9.25 wt.-% Y.sub.2O.sub.3 and 0.048 wt.-% Al.sub.2O.sub.3 .sup.3)contains 7.37 wt.-% Y.sub.2O.sub.3 and 0.048 wt.-% Al.sub.2O.sub.3 .sup.4)calculated as Fe.sub.2O.sub.3 .sup.5)calculated as Cr.sub.2O.sub.3 .sup.6)calculated as Pr.sub.2O.sub.3 .sup.7)calculated as Er.sub.2O.sub.3 .sup.8)calculated as La.sub.2O.sub.3

[0124] Subsequently, about 19 g of each of the coloured zirconia powders was introduced one after the other into the pressing die (diameter about 40 mm) of a laboratory axial compressor in four layers (at the bottom: L1, on top: L4) and densified axially under a pressure of about 160 MPa. The green body obtained in this way was debound and presintered using the following firing programme:

[0125] 60 K/min to 120 C.,

[0126] 24 K/min to 200 C.,

[0127] 10 K/min to 320 C.,

[0128] 60 K/min to 1050 C., holding time 3 h.

[0129] An about 2 mm thick section was sawn out of the debound and presintered blank obtained in this way using a saw (IsoMet 4000, Buehler, Esslingen). A paper or ruler was laid on the two lower outer edges of the section as a reference line and the area of maximum bending was measured under a stereo microscope (SZX 16, Olympus, Hamburg). The results are shown in FIGS. 5A (overview) and 5B (detailed view). The presintered blank showed only a negligible bending of 0.02 mm.

[0130] In addition, the development of the Vickers hardness HV.sub.5 over the layers was measured on the section of the presintered blank at 10 measuring points at a spacing in each case of 1.5 mm using a hardness testing machine (ZHU 0.2, Zwick Roell, Ulm). The hardness values determined are listed in the following table:

TABLE-US-00003 Measuring point HV.sub.5 [MPa] 1 583 2 591 3 593 4 584 5 586 6 652 7 623 8 636 9 605 10 597

[0131] From these values, a coefficient of distortion of d=0.114 was calculated.

[0132] Finally, the blank was densely sintered in a Programat S1 furnace (Ivoclar Vivadent AG, Schaan) using the following firing and cooling programme:

[0133] 600 K/h to 900 C., holding time 0.5 h

[0134] 200 K/h to 1500 C., holding time 2 h

[0135] 600 K/h to 900 C.,

[0136] 500 K/h to 300 C.

[0137] From the densely sintered blank obtained in this way, an about 2 mm thick section was in turn sawn out as described above and measured under the stereo microscope. The results are shown in FIGS. 6A (overview) and 6B (detailed view). The densely sintered blank showed no measurable bending.

Example 4

[0138] Aligning the Sintering Behaviour of a Four-Layer Blank Made of Zirconia Powders With Different Yttrium Content by Doping With La.sub.2O.sub.3

[0139] Example 3 was repeated with a larger batch. For this purpose, about 115 g of each of the coloured zirconia powders obtained as in Example 3 was introduced one after the other into the pressing die (diameter about 100 mm) of a powder press in four layers (at the bottom: L1, on top: L4) and densified axially under a pressure of about 160 MPa. The blanks were first of all debound and presintered as described in Example 3 and subsequently densely sintered. From the blanks obtained in this way, in each case about 2 mm thick sections were sawn out and measured under the stereo microscope. The results are shown in FIG. 7 (debound and presintered blank) and FIG. 8 (densely sintered blank in transmitted light). The blanks showed no measurable bending.

[0140] In addition, the development of the Vickers hardness HV.sub.5 over the layers was measured on the section of the presintered blank at 10 measuring points at a spacing in each case of 2 mm using a hardness testing machine (ZHU 0.2, Zwick Roell, Ulm). The hardness values determined are listed in the following table:

TABLE-US-00004 Measuring point HV.sub.5 [MPa] 1 581 2 588 3 599 4 600 5 589 6 625 7 634 8 620 9 606 10 613

[0141] From these values, a coefficient of distortion of d=0.088 was calculated.

Example 5

[0142] Aligning the Sintering Behaviour or a Blank With a Continuous Colour and Translucence Profile Made of Zirconia Powders With Different Yttrium Content by Doping With La.sub.2O.sub.3

[0143] For the preparation of coloured zirconia powders, commercially available zirconia powders (TOSOH TZ-PX-471 and TOSOH Zpex Smile) were used as raw materials and were treated with solutions of nitrate salts of colouring elements and optionally lanthanum as dopant according to the following table using the general process. Thereby, a powder (L1) suitable for a dentine layer and a powder (L2) suitable for a cutting layer were obtained:

TABLE-US-00005 Starting Powder powder Colouring elements.sup.1) Dopant L2 TOSOH 0.065 wt-% Fe.sup.4) Cutting Zpex Smile.sup.2) 0.00025 wt.-% Mn.sup.5) 0.0005 wt.-% Pr.sup.6) 0.0108 wt.-% Tb.sup.7) 0.24 wt.-% Er.sup.8) L1 TOSOH 0.092 wt.-% Fe.sup.4) 0.13 wt.-% La.sup.9) Dentine TZ-PX-471.sup.3) 0.00063 wt.-% Mn.sup.5) 0.0008 wt.-% Pr.sup.6) 0.0108 wt.-% Tb.sup.7) 0.5 wt.-% Er.sup.8) .sup.1)based on the total weight of the oxide mixture after the sintering .sup.2)contains 9.25 wt.-%Y.sub.2O.sub.3 and 0.048 wt.-% A1.sub.2O.sub.3 .sup.3)contains 7.37 wt.-% Y.sub.2O.sub.3 and 0.048 wt.-% A1.sub.2O.sub.3 .sup.4)calculated as Fe.sub.2O.sub.3 .sup.5)calculated as Mn.sub.2O.sub.3 .sup.6)calculated as Pr.sub.2O.sub.3 .sup.7)calculated as Tb.sub.2O.sub.3 .sup.8)calculated as Er.sub.2O.sub.3 .sup.9)calculated as La.sub.2O.sub.3

[0144] Subsequently, the coloured zirconia powders were introduced into the pressing die (diameter about 100 mm) of a powder press in the form of a continuous gradient using a suitable filling process and densified axially under a pressure of about 160 MPa. The green body obtained in this way was debound and presintered using the following firing programme:

[0145] 60 K; min to 120 C.,

[0146] 24 K/mm to 200 C.,

[0147] 10 K/min to 320 C.,

[0148] 60 K/min to 1050 C., holding time 3 h.

[0149] An about 2 mm thick section was sawn out of the debound and presintered blank obtained in this way using a saw (IsoMet 4000, Buehler, Esslingen). A paper or ruler was laid on the two lower outer edges of the section as a reference line and the area of maximum bending was measured under a stereo microscope (SZX 16, Olympus, Hamburg). The results are shown FIGS. 9A (overview) and 9B (detailed view). The presintered blank snowed only a negligible bending of 0.01 mm.

[0150] In addition, the development of the Vickers hardness HV.sub.5 over the gradient was measured on the section of the presintered blank at 10 measuring points at a spacing in each case of 1.5 mm using a hardness testing machine (ZHU 0.2, Zwick Roell, Ulm). The hardness values determined are listed in the following table:

TABLE-US-00006 Measuring point HV.sub.5 [MPa] 1 732 2 766 3 780 4 724 5 718 6 744 7 756 8 773 9 754 10 724

[0151] From these values, a coefficient of distortion of d=0.083 was calculated.

[0152] Finally, the section of the presintered blank was densely sintered in a Programat S1 furnace (Ivoclar Vivadent AG, Schaan) using the following firing and cooling programme:

[0153] 600 K/h to 900 C., holding time 0.5 h

[0154] 200 K/h to 1500 C., holding time 2 h

[0155] 600 K/h to 900 C.,

[0156] 500 K/h to 300 C.

[0157] The section of the densely sintered blank was in turn measured under the stereo microscope as described above. The results are shown in FIGS. 10A (overview) and 10B (detailed view). The densely sintered blank showed no measurable bending.

Example 6

[0158] Aligning the Sintering Behaviour or a Two-Layer Blank Made of Zirconia Powders With Different Yttrium Content by Doping With La.sub.2O.sub.3, Al.sub.2O.sub.3 and MgO

[0159] For the preparation of coloured zirconia powders, commercially available zirconia powders (TOSOH Zpex and TOSOH Zpex Smile) were used as raw materials and were treated with solutions of nitrate salts of colouring elements and optionally lanthanum or aluminium and magnesium as dopant according to the following table using the general process. Thereby, a powder (L1) suitable for a dentine layer and a powder (L2) suitable for a cutting layer were obtained:

TABLE-US-00007 Starting Powder powder Colouring elements .sup.1) Dopant L2 TOSOH 0.0676 wt-% Fe.sup.4) 0.03 wt-% Al.sup.9) Cutting Zpex Smile.sup.2) 0.0003 wt.-% Mn.sup.5) 0.01 wt-% Mg.sup.10) 0.0005 wt.-% Pr.sup.6) 0.0112 wt.-% Tb.sup.7) 0.2249 wt.-% Er.sup.8) L1 TOSOH 0.0734 wt.-% Fe.sup.4) 0.62 wt-% La.sup.11) Dentine Zpex.sup.3) 0.0006 wt.-% Mn.sup.5) 0.0001 wt.-% Pr.sup.6) 0.0095 wt.-% Tb.sup.7) 0.6647 wt.-% Er.sup.8) .sup.1) based on the total weight of the oxide mixture after the sintering .sup.2)contains 9.25 wt.-% Y.sub.2O.sub.3 and 0.048 wt.-% Al.sub.2O.sub.3 .sup.3)contains 5.36 wt.-% Y.sub.2O.sub.3 and 0.048 wt.-% Al.sub.2O.sub.3 .sup.4)calculated as Fe.sub.2O.sub.3 .sup.5)calculated as Mn.sub.2O.sub.3 .sup.6)calculated as Pr.sub.2O.sub.3 .sup.7)calculated as Tb.sub.2O.sub.3 .sup.8)calculated as Er.sub.2O.sub.3 .sup.9)calculated as Al.sub.2O.sub.3 .sup.10)calculated as MgO .sup.11)calculated as La.sub.2O.sub.3

[0160] Subsequently, about 10 g of each of the coloured zirconia powders was introduced one after the other into the pressing die (diameter about 40 mm) of a laboratory axial press in two layers (at the bottom: L1, on top: L2) and densified axially under a pressure of about 160 MPa. The green body obtained in this way was debound and presintered using the following firing programme:

[0161] 60 K/min to 120 C.,

[0162] 24 K/min to 200 C.,

[0163] 10 K/min to 320 C.,

[0164] 60 K/min to 1010 C., holding time 3 h.

[0165] An about 2 mm thick section was sawn out of the debound and presintered blank obtained in this way using a saw (IsoMet 4000, Buehler, Esslingen). A ruler was laid on the two lower outer edges of the section as a reference line and the area of maximum bending was measured under a stereo microscope (SZX 16, Olympus, Hamburg). The results are shown in FIGS. 11A (overview) and 11B (detailed view). The presintered blank showed only a negligible bending of 0.03 mm.

[0166] Finally, the section of the presintered blank was densely sintered in a Programat S1 furnace (Ivoclar Vivadent AG, Schaan) using the following firing and cooling programme:

[0167] 600 K/h to 900 C., holding time 0.5 h

[0168] 200 K/h to 1500 C., holding time 2 h

[0169] 600 K/h to 900 C.,

[0170] 500 K/h to 300 C.

[0171] The section of the densely sintered blank was again measured under the stereo microscope as described above. The results are shown in FIGS. 12A (overview) and 12B (detailed view). The densely sintered blank showed no measurable bending.

Example 7

Comparison

[0172] Sintering Behaviour of a Two-Layer Blank Made of Zirconia Powders With Different Yttrium Content Without Doping With La .sub.2O.sub.3

[0173] Example 6 was repeated identically, wherein however no dopant (lanthanum, aluminium, magnesium) was added. The results for the debound and presintered blank are shown in FIGS. 13A (overview) and 13B (detailed view). Therein, it can be clearly seen that the dentine layer (L1) has shrunk more than the cutting layer (L2), whereby the blank has obtained a convex shape. The bending of the blank was 0.74 mm. The results after the dense sintering of the blank are shown in FIGS. 14A (overview) and 14B (detailed view). According to this, even the densely sintered blank still showed a bending of 0.05 mm.

[0174] Example 6 was repeated in identical manner. However, no dopant (lanthanum, aluminium, magnesium) was added. The results for the debound and pre-sintered blank are shown in FIGS. 13A (overview) and 13B (detailed view).

Example 8

[0175] Aligning the Sintering Behaviour of a Blank With a Continuous Colour and Translucence Gradient Made of Zirconia Powders With Different Yttrium Content by Doping With La.sub.2O.sub.3, Al.sub.2O.sub.3and MgO

[0176] For the preparation of coloured zirconia powders, commercially available zirconia powders (TOSOH Zpex and TOSOH Zpex Smile) were used as raw materials and were treated with solutions of nitrate salts of colouring elements and optionally lanthanum or aluminium and magnesium as dopant according to the following table using the general process. Thereby, a powder (L1) suitable for a dentine layer and a powder (L2) suitable for a cutting layer were obtained:

TABLE-US-00008 Starting Powder powder Colouring elements .sup.1) Dopant L2 TOSOH 0.0676 wt-% Fe.sup.4) 0.03 wt-% Al.sup.9) Cutting Zpex Smile .sup.2) 0.0003 wt.-% Mn.sup.5) 0.01 wt-% Mg.sup.10) 0.0005 wt.-% Pr.sup.6) 0.0112 wt.-% Tb.sup.7) 0.2249 wt.-% Er.sup.8) L1 TOSOH 0.0734 wt.-% Fe.sup.4) 0.62 wt-% La.sup.11) Dentine Zpex.sup.3) 0.0006 wt.-% Mn.sup.5) 0.0001 wt.-% Pr.sup.6) 0.0095 wt.-% Tb.sup.7) 0.6647 wt.-% Er.sup.8) .sup.1) based on the total weight of the oxide mixture after the sintering .sup.2)contains 9.25 wt.-% Y.sub.2O.sub.3 and 0.048 wt.-% Al.sub.2O.sub.3 .sup.3)contains 5.36 wt.-% Y.sub.2O.sub.3 and 0.048 wt.-% Al.sub.2O.sub.3 .sup.4)calculated as Fe.sub.2O.sub.3 .sup.5)calculated as Mn.sub.2O.sub.3 .sup.6)calculated as Pr.sub.2O.sub.3 .sup.7)calculated as Tb.sub.2O.sub.3 .sup.8)calculated as Er.sub.2O.sub.3 .sup.9)calculated as Al.sub.2O.sub.3 .sup.10)calculated as MgO .sup.11)calculated as La.sub.2O.sub.3

[0177] Subsequently, the coloured zirconia powders were filled into the pressing die (diameter about 100 mm) of a powder press in form of a continuous gradient using a suitable filling process and densified axially at a pressure of about 160 MPa. The green body obtained in this way was debound and presintered using the following firing programme:

[0178] 60 K/min to 120 C.,

[0179] 24 K/min to 200 C.,

[0180] 10 K/min to 320 C.,

[0181] 60 K/min to 1050 C., holding time 3 h.

[0182] An about 2 mm thick section was sawn out of the debound and presintered blank obtained in this way using a saw (IsoMet 4000, Buehler, Esslingen). The result is shown in FIG. 15. The presintered blank showed only a negligible bending.

[0183] In addition, the development of the Vickers hardness HV.sub.5 over the gradient was measured on the section of the presintered blank at 10 measuring points at a spacing in each case of 2 mm using a hardness testing machine (ZHU 0.2, Zwick Roell, Ulm). The hardness values determined are listed in the following table:

TABLE-US-00009 Measuring point HV.sub.5 [MPa] 1 651 2 670 3 676 4 673 5 642 6 600 7 557 8 543 9 560 10 554

[0184] From these values, a coefficient of distortion of d=0.22 was calculated.

[0185] Finally, the section of the presintered blank was densely sintered in a Programat S1 furnace (Ivoclar Vivadent AG, Schaan) using the following firing and cooling programme:

[0186] 600 H/h to 900 C., holding time 0.5 h

[0187] 200 K/h to 1500 C., holding time 2 h

[0188] 600 K/h to 900 C.,

[0189] 500 K/h to 300 C.

[0190] The section of the densely sintered blank was again measured under the stereo microscope as described above. The result is shown in FIG. 16. The densely sintered blank showed only a negligible bending.